Electrical resistivity dip meter



E. v. MURPHREE ELECTRICAL RESISTIVITY DIP METER Oct. 13, 1953.

Filed May 12, 1951 3 Sheets-Sheet 1 E52! wmurphree, Snvumm- QLor1er G.ia,

1953 E. v. MURIPHREE ELECTRICAL RESISTIVITY DIP METER 3 Sheets-Sheet 2Filed May 12, 1951 GALVANOMETEQ AMOLIFIEQ H9 at \rnurphr 6e,

Snvzqbar Otrborneg OCt- 13, V. MURFEHREE ELECTRICAL RESISTIVITY DIPMETER I Filed May 12, 1951 3 Sheets-Sheet 3 Ely-er murphre e, :5 av c anor 7 Clttornezqs Patented Oct. 13, 1953 ELECTRICAL RESISTIVITY DIP METEREger V. Murphree, Summit, N. J., assignor to Standard Oil DevelopmentCompany, a corporation of Delaware Application May 12, 1951, Serial No.226,039

10 Claims.

1 This invention concerns. an improved apparatus for determining the dipand strike of strata traversed by a bore hole. In accordance with thisinvention, variations in electrical resistivity occurring at theboundary of different strata in the bore hole are employed to provideinformation, as to the attitude of the strata. This is accomplished bydetermining the properties of elec trical resistivity along at leastthree profiles of the bore hole while simultaneously determining theazimuthal orientation of the electrodes employed for this purpose. It isa particular feature of this apparatus that the necessary determinationsof electrical resistivity are made by employing a stationary fieldtechnique. According to this technique, a current electrode ismaintained stationary in the bore hole while potential electrodes aremoved along the bore hole while the necessary determinations are made.

The practice of electrical logging has been well developed and widelyemployd in the examination of bore holes in the attempt to locate andproduce petroleum. A great variety of electrode arrangements have beensuggested for the general purpose of determining electrical propertiesof strata through which bore holes pass. The present invention isparticularly adapted for use with the eelctrode system generally knownas the three electrode system. A three electrode system is described,for example, in the patent to Mounce, No. 2,376,168, granted May 15,1945.

- The three electrode system employs what is known as a "currentelectrode positioned in the bore hole together with two potentialelectrodes which are also positioned in the bore hole. In reality afourth electrode is also employed which is grounded at the surface ofthe ground-as by placinggit in theslush pit associated with the mudcirculation system employed in the well. This fourth electrode may alsobe called a current electrode. A source of electrical potential isconnected between the current electrode at the surface of the ground andthe current electrode which is lowered into the bore hole. setting up aflow of current through the earth between the two current electrodes.The two potential electrodes, positioned in the bore hole, are employedto measure the potential difference existing adjacent the bore hole dueto current flowing between the current electrodes. Conventionally, thedown-the-hole current electrode, and the two potential electrodes arespaced apart on a common casing and moved along the bore hole in orderto log the resistivity characteristics of the bore hole. The presentinvention is concerned with an adaptation of this conventional threeelectrode system permitting a determination of the dip and strike ofstrata.

A difiiculty of the three electrode logging system of the natureheretofore indicated is that distortion appears in the determinedresistivity characteristics of a bore hole, making it difiicult tointerpret the information. The primary factor causing distortion is thatpassage of the current electrode, as well as the potential electrodespast a high resistivity stratum causes variations in the potentialdifference recorded by the potential electrodes. Since it is desirableto determine resistivity changes occurring only due to movement of thepotential electrodes past a boundary of diifering resistivity, thealterations which occur on passage of the current electrode past such aboundary result in resistivity data which is diflicult to interpret. Asa result of the distortion caused by this factor, the type of logobtained in conventional three eelctrode logging systems disqualifiesuse of such systems for the purpose of making dip and strikedeterminations.

It has now been discovered that the logging record obtained in a threeelectrode system can be materially improved and simplified by employingwhat is known as the stationary field technique. In this technique thedown-the-hole current electrode is maintained stationary while thepotential electrodes are moved either actually or effectively. Thus, forexample, the potential electrodes may be suspended below the currentelectrode by a winch arrangement. Logging may then be conducted in astepwise fashion by placing the current electrode at a given depth inthe well and by raising the potential electrodes on the wincharrangement toward the current electrode. Thereafter, this procedure isrepeated at different levels. This technique eliminates distortion ofthe resistivity log due to movement of the current electrode pastboundaries of differing resistivities during the determinations. As aresult, the stationary field technique provides electrical logs whichare well suited for use in an apparatus for determining the dip andstrike of strata.

To understand the basic principles on which this invention operates,consider the logging of a perpendicular bore hole. If three pairs of po.tential electrodes are spaced around the wall of a bore hole and aremoved along the bore hole, in conjunction with properly positionedcurrent electrodes, to obtain three resistivity profiles of the borehole, it is apparent that information will be given indicating whetherstrata boundaries encountered are perpendicular to the bore hole or areinclined -to the bore hole. Thus, if the three pairs of potentialelectrodes are moved past a boundary of differing resistivity lying in ahori zontal plane, each of the electrodes will reach this boundary,atthe same instant and provide an alteration in resistivity readings atthe same instant. If, however, such a boundary is inclined with respectto the bore hole, then the three pairs of electrodes will sequentiallyreach the boundary so that the resistivity changes recorded by each pairof electrodes will occur at a different instant. It is this principlewhich is employed in accordance with this invention.

In a preferred embodiment of the invention, one current electrode isgrounded at the surface of the earth according to conventional practice.The other current electrode is positioned on a cable and is lowered intothe bore hole, to be maintained at any desired depth therein. Threeseparate pairs of potential electrodes are urged against the bore holeby caliper arms supported by a. casing which is independently suspendedabove or below the housing of the down-the-hole current electrode. Forexample, a winch arrangement may be provided in a housing supporting thecurrent electrode to raise or lower the casing supporting the potentialelectrodes: the potential electrodes may thereby be moved with respectto the current electrode. Or again, one cable from the surface of theearth may be used to suspend the down-the-hole current electrode, whilea second cable may be used to suspend the casing carrying the threepairs of potential electrodes. The potential electrodes are supported ina manner permitting the three pairs of electrodes to be forced againstthe wall of the bore hole at points which are preferably spaced 120degrees apart with respect to the axis of the bore hole. Preferably eachof the two potential electrodes making up a given pair of electrodes,are maintained in vertical alignment and are spaced apart a few inchesto about one foot. This arrangement of electrodes essentially providesthree pairs of electrodes spaced around the bore hole in two planesperpendicular to the axis of the bore hole so that the electrodes ofeach pair are in vertical alignment and displaced by about one foot. Bymoving these pairs of electrodes along the bore hole, each of the pairsof electrodes will provide an electrical resistivity profie of theportion of the bore hole along which it is moved. By a comparison of thedeterminations of the three pairs of electrodes, the sequence at whichthe pairs contact strata boundaries may be determined. This inventionmay be used to precisely indicate the dip and strike of strataintersecting the bore hole by correlating this information with otherdata which may be obtained.

Thus, it is necessary to know the azimuthal orientation of each of theelectrodes at any point in the well at which dip information is desired.Due to torsional forces acting to rotate the electrodes in the bore holeas they are raised or lowered, it is necessary to employ apparatus whichwill at all times indicate in which direction the electrodes may haverotated. Azimuthal orientation devices are now known to the art which ybe pl y d for this purpose.

A in f r precise dip determinations it is necessary that the diametricaldisplacement of he cal p s forcing the electrodes against the bore holebe known. Due to erosion an ot er factors, the diameter of the bore holemay vary substantially along its length. Consequently, it is necessaryto essentially determine the diameter of the bore hole at any time atwhich a dip determination is required. This data may be provided bycausing the calipers supporting the potential electrodes to developelectrical signals to continuously indicate the extension of thecalipers, or, in other words, to indicate the diametrical displacementof the electrodes in the bore hole.

It is also necessary that precise depth measurements be available toindicate the depth at which the potential electrodes and the currentelectrode are suspended during dip determinations. The informationrequired for this purpose may be obtained from conventional depthdeterminations of the housing carrying the current electrodes and thecasing carrying the potential electrodes. This information may beobtained employing a conventional measuring wheel on the cablesupporting the housing of the current electrode and by information as tothe operation of the winch suspending the potential electrodes.

In the event, as is often the case, that the bore hole is not vertical,but somewhat inclined, it is 'also necessary that information beobtained as to the extent and direction of incline of the bore hole.This information may be obtained by reference to a conventionallyconducted inclination log of the bore hole. Alternatively, aninclinometer may be made an intergral part of the apparatus of thisinvention.

A preferred embodiment of this invention is diagrammatically illustratedin the accompanying drawings, in which:

Figure '1 shows an elevational section of the down-the-hole portion ofthe dip determining apparatus of this invention; and

Figure 2 schematically indicates a wiring circuit for the principalcomponents of the apparatus; and

Figure 3 diagrammatically illustrates diiferent types of electrical logsincluding the traces obtained in accordance with this invention.

Referring now to Figure 1, the apparatus illustrated is lowered in abore hole identified by numeral I. The apparatus may consist of asuitable housing 2 provided with a current electrode 3, which mayconsist of a ring extending circumferentially around the casing 2 andinsulated therefrom. Electrode 3 comprises the current electrode to bemaintained at different levels within the bore hole as formerlyindicated. The housing 2 includes a reel 4 driven by a Selsyn motor 5.The motor 5 is arranged to drive the reel 4 through a, suitable speedreduction gear box 6 and bevel gearing 1. Operation of reel 4 controlsthe distance at which the apparatus 8 is suspended below housing 2 oncable 9. Casing 8 provides support for the three pairs of potentialelectrodes required. For simplicity, the drawing of Figure 1 shows onlytwo pairs of potential electrodes. As shown by Figure 1a, three pairs ofpotential electrodes are actually employed which are equally spacedabout the circumference of the apparatus. Thus the potential electrodescomprise a first pair 9 and I0, and a second pair H and I2. Each of theelectrodes are fixed on a caliper arm 13 which is pivotally supported byeasing 8 and is urged against the bore hole by spring I4. The electrodesare preferably disc-shaped and are imbedded in an insulating material asby molding or vulcanizing them in place so that no liquid may contact anelectrode at any place other than at its exposed face. Caliper arm I3 isarticulated adjacent electrode 9 or H, so that the lower portion of thecaliper may be urged against the bore hole by spring 32. This insurescontact of both electrodes l and I2 with the wall of the bore hole. Forsimplicity the electrical connections to the electrodes have not beenshown in Figure 1, it being understood that suitable insulatedconductors extend from each of the electrodes into the casing 8 andthrough cable 9 to housing 2. Cable 9 is arranged on reel 4 so that sliprings IS on the shaft of reel 4 may transmit the readings of theelectrodes through suitable brushes l6 connected to insulated conductorsin the cable I! leading to the surface of the ground. Since each of thecaliper arms l3 are urged forcibly against the bore hole, each of thepotential electrodes 9 to l2 will be forced through the mud of the borehole in proximity or in direct contact with the strata forming the wallof the bore hole. This minimizes interference in resistivity readingscaused by the presence of drilling mud in the bore hole. The caliperarms are of suitable dimensions so that electrodes 9 and i0 making upone pair of electrodes are vertically displaced by about 2 to 12 inches,while electrodes H and I2 comprising the second pair of electrodes aresimilarly displaced.

Information as to extension of each of the caliper arms may be providedby the means diagrammatically illustrated. A slender drive rod [8 may bepivotally fixed to each of the caliper arms l3. The other end of thedrive rods 18 may terminate in an armature core l9 disposed within asolenoid coil 20. As a consequence of this arrangement as a caliper armI3 is moved inwardly or outwardly by contact with the bore hole, thearmature l3 will be moved into or out of the field of the solenoid coil.Consequently, by determining the inductance of each of the solenoidcoils, the extension of each of the calipers may be ascertained. Again,for simplicity, the electrical conductors extending from each solenoidupwardly to reel 4 establishing an electrical circuit through thebrushes and thence to the conductors of cable ll have not beenillustrated. It is apparent that many alternative methods may beemployed for determining the extension of the calipers and use of thesolenoid coils as indicated is only intended for exemplary purposes.

The arrangement of caliper arms described, forcing the electrodesagainst the bore hole has the important function of maintaining theentire casing 8 in general alignment with the bore hole and essentiallyat the center of the bore hole This follows from the fact that if one of"the caliper arms were to be greatly compressed.

the resulting tension on its associated spring l4 would cause shiftingof the entire casing 8 toward the opposite side of the bore hole toequalize tension on each of the springs 4. This effect insures theapparatus maintaining a central position in the bore hole. Similarly, itfollows that the springs 32 associated with the articulated portion ofeach caliper arm co-act with springs Hi to the apparatus in substantaialalignment with the bore hole.

Casing 8 provides a protective housing for other apparatus which may beemployed. Thus, it is necessary that an orientation indicator 4| beprovided in casing 8. Any of the orientation devices known to the artmay be employed. For example, the apparatus disclosed and claimed in U.S. Patent No. 2,332,777, issued to Boucher in October 26, 1943 may beemployed. As this element is no part of the present invention, it willnot be described in detail. It will be understood however, that theorientation instrument 4| is of a nature to provide information as toazimuthal position of the casing 8, and consequently of each of thecalipers and each of the electrodes associated with the casing.

In addition to the apparatus specifically illus trated in Figure 1, itis to be understood that a suitable measuring wheel is to be associatedwith the apparatus at the surface of the earth to de termine the depthof the casing 2 in the bore hole. As described therefore, the apparatus"of Figure 1 provides information as to the potential difference betweenthe pairs of electrodes carried by the casing 8 and suspended fromhousing 2. In addition, electrical information will be provided at thesurface of the earth in regard to the extension of the caliper arms andas to the azimuthal orientation of the casing 83.

The circuit diagram for the principal electrical components of theapparatus of Figure l is illustrated in Figure 2. In Figure 2 allportions of the circuit below the line 2I-2l indicates the down-the-holeportion of the apparatus, while the portion of the circuit above line2|-2I is associated with apparatus at the surface of the earth.Referring to Figure 2, a Selsyn motor 22 at the surface of the earth isdriven in synchronism with the Selsyn motor 5 positioned in casing 6 inthe bore hole. Switch 42 controls the starting and stopping of themotors as described. It will be understood that as an alternative tothis arrangement the reel may be operated by a conventional motor sothat the Selsyn 5 attached to the reel will cause recorder Selsyn 22 toturn proportionately to the reel. A source of alternating current 23 hasone terminal grounded while the other terminal is connected to anelectrical conductor 24 supplying current to the field coils of both ofthe Selsyn motors and also supplying current to the current electrode 3located on the housing 2 in the bore hole. The Selsyn motor circuit thusconstitutes a control circuit for correlating the changes in distancebetween the current electrode 3 and the potential electrode supported oncasing 8. Thus operation of Selsyn motor 22 associated with a recordingpaper or film serves to simultaneously operate Selsyn motor 5 in amanner to raise casing 8 or reel 4 towards housing 2.

Each of the potential electrodes on casing B such as electrodes 9 and I0is electrically connected to a vacuum tube measuring apparatus 25associated with a galvanometer 26. The gal vanometer 26 may be of thenature controlling the position of a mirror so as to effect the positionat which reflected light will impinge on photographic film moved bySelsyn motor 22. As a result, a record will be obtained as to thepotential readings of each of the three pairs of electrodes carried byeasing 8. In addition, it is understood that means are associated withthe record to simultaneously indicate the orientation of easing 8 andthe depth at which the potential electrodes are positioned in the borehole.

Referring now to Figure 3 traces of the nature obtained in threeelectrode systems are illustrated. The curve shown as Figure 3-1)indicates the type of log obtained in the conventional three electrodesystem, showing the trace provided by a single pair of potentialelectrodes. For comparative purposes Figure 3-a shows the aesacse 1appearance of what would be an ideal electrical log. In comparing curves,3-a and 3-22, it will be noted that the conventional electrode systemis not effective in clearly indicating the location of boundariesbetween strata. Finally, Figure 3-0 illustrates the type of loggingrecord obtained with the apparatusfiescribed. In Figure 3-0 records ofeach of the three pairs of electrodes employed are indicated.

It will be noted that the curves obtainable reasonably approximate theideal logging record of Figure 3-a. Each of the separate groups ofcurves indicated in Figure 3-0 correspond to a stepwise movement of thelogging apparatus in the bore hole. As illustrated, each of these curvesslopes upwardly and to the right since as the distance between thepotential electrodes and the current electrode is altered, the potentialdifference varies as a square of this change in distance. It is for thisreason that the distance to which the potential electrodes are suspendedbelow the current electrode becomes important. Because of this factorthe potential electrodes must not be more than about 30 feet below thecurrent electrode as the maximum. The uppermost of the three comparativecurves of Figure 3-0 indicate that each of the pairs of electrodesencountered a boundary at the same instant. showing this boundary wasperpendicular to the axis of the bore hole. The next lower of the groupof curves in Figure 3-0 shows a difference in the contact time of asecond boundary between the diflerent pairs of electrodes indicatingthis boundary was inclined with respect to the bore hole. By correlatingthe data of Figure 3-0 with orientation information showing theazimuthal position of each of the electrodes, it is thus possible toaccurately determine the dip and strike of the boundaries indicated.

A convenient and valuable modification of the apparatus describeddepends upon a correction for the varying distance between thestationary current electrode and the moving potential electrodes. Bysuitably correcting for this factor, a curve such as 3-11 may beobtained. This may readily be accomplished by positioning alogarithmically wound potentiometer on the shaft of Selsyn motor 22. Thevoltage taps of this potentiometer are to be connected to conductor 24leading to the down-the-hole current electrode 3. Consequently, onrotation of Selsyn motor 22, affecting the distance of the potentialelectrodes below the current electrodes, the amount of current reachingthe current electrode will automatically be corrected to compensate forthe varying distance between the current electrode and the potentialelectrodes.

8 tial electrodes may be eliminated by various mod ifications. Forexample, by employing a number of potential electrodes verticallyaligned in the bore hole and by switching two diflerent pairs of theseelectrodes operating at different levels in the bore hole it is possibleto secure the constructive movement of the potential electrodes relativeto the current electrode without necessity for the actual movement ofthese electrodes.

What is claimed is:

l. A clip and strike determining apparatus for logging of bore hole,comprising in combination a housing carrying a current electrodesupported in said bore hole, a casing positioned below said housingmeans within said housing adapted to vary the position of the saidcasing below said housing, said casing being provided with at leastthree caliper arms substantially spaced from each other about thecircumference of the casing, said caliper arms including means to urgethem outwardly of the casing, a pair of electrodes positioned on each ofsaid calipers, and independent electrical circuit connections betweeneach of said three pairs of electrodes and recording apparatus at thesurface of the earth.

2. The apparatus defined by claim 1 in which the said means to controlthe position of the easing below the housing comprises a Selsyn motor Asformerly indicated, to secure precise dip and strike data accounting forvarying inclinations of the bore hole, it is valuable to include aninclinometer in the casing 8 supporting the potential electrodes. Theinclinometer must be of the nature capable of indicating both the extentand direction of inclination of casing 8. An inclinometer of the typedisclosed in U. S. Patent No. 2,365,999, issued to Boucher, December 26,1944, may be adapted for use in this invention. It is necessary tosynchronize the operation of the inclinometer with the general type oforientation indicator formerly described.

In the embodiment of the apparatus illustrated and particularlydescribed, the potential electrodes a e actually moved with respect tothe stationary current electrode during determinations. The necessityfor actual movement of the potenand in which the said recordingapparatus includes a synchronized Selsyn motor whereby movement of therecording apparatus indicates the depth below the said housing at whichthe said pairs of electrodes are located.

3. The apparatus defined by claim 1 including means to determine theazimuthal orientation of the said casing.

4. The apparatus defined by claim 1 in which the electrodes comprising apair of electrodes are spaced apart on each caliper arm by a distance ofabout 2 to 12 inches.

5. Apparatus for determining the dip and strike of strata traversing abore hole comprising means for maintaining a first electrode in a fixedposition within said bore hole, means for positioning at least threepairs of electrodes at a distance less than about 30 feet from saidfirst electrode, said pairs of electrodes being spaced substantiallyapart from each other around the circumferences of the bore hole, andmeans for varying the effective distance between said three pairs ofelectrodes and said first electrode.

6. The apparatus defined by claim 5 in which each pair of the said threepairs of electrodes comprises two electrodes maintained in substantiallyvertical alignment and spaced from each other.

7. The apparatus defined by claim 5 including means to force each of theelectrodes making up the said three pairs of electrodes against the borehole wall.

8. The apparatus defined by claim 5 in which the said means forpositioning the three pairs of electrodes includes means to determinethe azimuthal orientation of said pairs of electrodes.

9. The method of determining the dip and strike of strata traversing abore hole in which a first electrode is sequentially maintained atdifferent fixed positions in the bore hole and in which three pairs ofelectrodes spaced substantially apart from each other around thecircumference of the bore hole are efi'ectively moved with respect tothe said first electrode while said first electrode is maintained in afixed position and simultaneously exhibiting the depth of the said pairsof electrodes in the borehole, the azi- EGER V. MURPHREE.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Kinley Dec. 14, 1937 Athy et a1 Nov. 11, 1941 Haynes Jan. 12,1943 Robicloux Feb. 8, 1944 Chun Jan. 15, 1946 Archie May 21, 1946 EnnisJan. .14, 1947 Johnson Feb. 11, 1947 D011 Sept. 23,1947 Doll July 12,1949 .Brandon Apr. 4, 1950 Hildebrandt May 8, 1951

